BSC support for CBSP (cell broadcast service protocol)

Right now, the only way to send cell broadcast messages is to use the BSC VTY.

3GPP Specifies CBSP in TS 48.049 as the protocol used between BSC and CBC to configure which kind of messages shall be broadcast where, and for how long.

It's a simple TLV based protocol encapsulated in TCP, so the protocol itself should be rather easy to support. However, it requires an underlying SMSCB scheduler that provides the required functionality for routing/scheduling. See #3536.